The present invention concerns a device for welding together a cover and a vessel of thermoplastic material. The invention concerns a tool for so-called hot plate welding or mirror welding, i.e., a process wherein the surfaces that are to be joined are heated to a molten state and thereafter brought together, suitably under controlled pressure, so that a bond is attained between the two parts. The tool according to the invention is especially suited for use when the vessel or cover presents elements or protrusions that extend up above or down below the respective edges, so that a welding tool that is flat and covers the whole surface cannot be used. It is anticipated that the tool will find its primary application in the welding of the cover onto a vessel for cells of electric storage batteries.
Hot tool welding is a method that is much utilized for joining vessels and covers of thermoplastic material when the wall thicknesses are about 1 mm or more. Automatic machines are available for execution of this welding operation. The tool, i.e., the heating plate from which heat is transferred to the surfaces that are to be joined, consists in these machines of a flat plate with the same configuration as the surfaces or edges that are to be joined. Such machines have also been used for the fastening of covers on battery vessels. In certain types of batteries there are connecting elements in the electrical system that stick up beyond the vessel edge. The welding tool is then given recesses that correspond to these elements. In welding together covers and vessels, these elements and recesses are first brought into a specific position with reference to each other, at a suitable spaced distance, so that the welding tool can be introduced between them. The vessel and cover are then applied against the tool which, as a rule, is managed by holding the cover stationary and bringing the welding tool up against it. Thereafter the vessel is brought up against the underside of the welding tool. When a sufficient amount of heat has been transferred, the vessel is lowered and then the tool, whereafter the tool is withdrawn, i.e., it executes a motion that corresponds at least to the width of the narrowest vessel wall. The vessel is then moved upward so that it is applied against the cover and is pressed against it with a specific pressure, for a specific period of time to effect the bonding.
There are a number of drawbacks associated with this known method. A substantial disadvantage is that the welding tool is substantially bigger than the edge surfaces that are to be heated. Hereby there are unnecessary heat losses, and because the tool covers the whole surface of the vessel, the recesses mentioned above have to be made in the tool, to accommodate any elements that extend upward from the vessel. Moreover, the welding tool has to execute a motion in two directions, one of which motions is relatively long, i.e., removing the tool from between the cover and vessel. This has the effect that the time elapsing between the breaking of contact between the tool and the edges of the vessel and cover and the pressing of these surfaces against each other as described above can amount to about 2 seconds. During this time, there is a certain cooling off of the heated thermoplastic material which can have a negative effect on the welding and on the properties of the obtained weld. It would therefore be highly valuable if this time could be reduced. Another disadvantage of the known methods is that there has to be a special welding tool for each configuration of the surfaces that are to be welded. This entails extra labor and time expenditure in switching between various types of vessels and covers.